Earth System Science

Earth contains land, air, water, and life phenomena with rhythms of natural variability that are connected over time and space (Plate 1). For example, continental collisions produce jagged mountains that gradually erode into rounded ranges. With weathering of the rocks, minerals are transported into streams that flow into rivers which add salts to the sea. In the ocean and on land, biological assemblages flourish and disappear with habitat changes that occur during their lifetimes as well as over millennia and millions of years. The challenge at all scales in this interconnected Earth system is to interpret relationships among seemingly disparate natural phenomena, events, and entities.

The basic context of the Earth system is the planet itself (Fig. I), third from the Sun in our solar system, with dimensions from its inner molten core outward to the edge of its atmosphere (Chapter 1: Global Dimensions). Earth is unique within our solar system because of liquid water, which is the principal ingredient for life as we know it. The largest water reservoir is the ocean, which circulates around the continents and interacts with the land largely through precipitation and runoff. Outside of the ocean—which contains more than 95 percent

(single view) map projection of the Earth showing the continents and oceans at different degrees of latitude, which parallel each other from 90° north through the equator at 0° to 90° south. Perpendicular to the latitudes are the longitudes from 180° east through the Greenwich meridian at 0° to 180° west (based on the 360 degrees in a circle; longitudes all converge at polar points on a sphere). At latitudes higher than 60° south, Antarctica is surrounded by water at all longitudes. This illustration, which was generated with Xearth Web software (http://ecco.bsee.swin.edu.au/chronos/xearth), is oriented in relation to the 23.5 degree tilt of the Earth's axis. Signatories to the Antarctic Treaty (dots) are shown to illustrate the international dimensions of human interest in Antarctica (Chapter 5: International Stewardship).

of the water—most of the water on Earth is locked into ice, and most of the ice is in Antarctica.

Life itself began evolving in the primordial soup of early Earth nearly a billion years after the planet originated around 4.5 billion years ago. Taking energy from sunlight, plants progressively added oxygen to the atmosphere—making the planet habitable for animals, which consumed the plants and each other. Species' responses to habitat change, like the growth of minerals in rocks, scribe the history of Earth.

Imagine looking at the Earth from outer space. What questions would you ask about the blue sphere underneath, and how would you answer them? Our world is complex, and there are many ways to develop natural and social science perspectives on the Earth system. However, in all cases, educational approaches are based on questions (Chapter 2: Conceptual Integration). Inquiry is at the heart of learning and is fundamental to studying the Earth as a system of interacting natural phenomena that affect and are affected by human civilization.

A common feature of questions involves relating events, entities, and phenomena to effectively address the who, what, when, where, why, and how of things. Symbols such as interlocking circles are often used as simple descriptions of how diverse topics intersect. Similarly, arrows between elements in a diagram often are used to portray pathways connecting reservoirs in systems, such as the Earth.

A most basic first principle in science is the twinned concept of time and space. Each event has an impact that propagates over distance and time as far as its initial magnitude allows. Gradients— which are commonly represented by the axes of a graph—are widely used to describe the relative dimensions of events, entities, or phenomena. For example, a timeline could portray your height at different ages or the Earth's temperature during different millennia.

Hierarchies—which are commonly represented by outlines—can be used to further describe relationships among discrete categories in a system across levels of complexity. For example, your home on Earth can be located in relation to the continents with their nations, states, counties, cities, and streets. Similarly, life forms can be distinguished in relation to their kingdom, phylum, class, order, family, genus and species (mnemonic: ''King Philip Can Order Fresh Green Spinach'').

The ultimate tool for integrating information is decision-making. Deciding how things work—in the present as well as in the past or future—requires information from different sources. When Sir Isaac Newton's apple fell, he had to know something about the Earth as well as his falling fruit before he could make the leap to a theory of gravity. Decisions also take form through group activities, as in national leg islatures or Antarctic Treaty Consultative Meetings, where practical solutions are formulated for society at all levels. In making decisions about the Earth and humankind, progress involves global perspectives and information that are used for designing century-scale policies that will enlighten our civilization into the future.